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Path: ...!2.eu.feeder.erje.net!3.eu.feeder.erje.net!feeder.erje.net!weretis.net!feeder8.news.weretis.net!reader5.news.weretis.net!news.solani.org!.POSTED!not-for-mail From: Gerhard Hoffmann <dk4xp@arcor.de> Newsgroups: sci.electronics.design Subject: Re: noise question Date: Tue, 16 Jul 2024 01:40:29 +0200 Message-ID: <v74c1d$kdm1$1@solani.org> References: <82ca9jl6d27llb1m5gagmlteop59p3cc0a@4ax.com> <v73fem$o96t$1@dont-email.me> <4aha9j9es0d8bhvforetas77fiatfr8f5q@4ax.com> <v73mda$pp21$1@dont-email.me> <puoa9jdibsc7nqsfpjk5s0m8c427c0gjca@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Mon, 15 Jul 2024 23:40:29 -0000 (UTC) Injection-Info: solani.org; logging-data="669377"; mail-complaints-to="abuse@news.solani.org" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:SkQvB+gT29frb/k9b/kOd4O/vKs= Content-Language: en-US X-User-ID: eJwNyskRACEMA7CWnMVxhnIgR/8lsHrLl0wZlIs+Pkaixr6dh3d1GxCJToqbgbKKK6T5/O3AHxNKEOE= In-Reply-To: <puoa9jdibsc7nqsfpjk5s0m8c427c0gjca@4ax.com> Bytes: 4272 Lines: 83 Am 15.07.24 um 20:04 schrieb john larkin: > On Mon, 15 Jul 2024 19:33:52 +0200, Jeroen Belleman > <jeroen@nospam.please> wrote: > >> On 7/15/24 18:09, john larkin wrote: >>> On Mon, 15 Jul 2024 17:35:08 +0200, Jeroen Belleman >>> <jeroen@nospam.please> wrote: >>> >>>> On 7/15/24 16:30, john larkin wrote: >>>>> Does a negative 50-ohm resistor make as much noise as a regular 50 ohm >>>>> resistor? >>>>> >>>>> I'd sorta guess the current noise to be the same, and maybe the >>>>> open-circuit voltage noise is infinite. >>>>> >>>>> I could Spice that, at least the current noise, if Spice handles it >>>>> right. LT Spice noise analysis is kind of weird. >>>>> >>>> >>>> I just tried it: In LTspice the sign doesn't matter, >>>> only the absolute value. Also, if you put a positive >>>> resistor in series with negative one, the noise >>>> voltages add RMS-wise, like you'd expect of independent >>>> sources. >>> >>> Cool. Thanks. >>> >>>> >>>> In real life, a negative resistor may have more or >>>> less noise than an actual resistor, depending on the >>>> low-noise design skills of the designer. >>>> >>>> I think you knew that... >>>> >>>> Jeroen Belleman >>> >>> Sure, I was considering an ideal neg resistor, without added noise >>> from active parts. >>> >>> As a college project, I built a 2-terminal negative resistor and >>> plugged the negative value into a bunch of equations (voltage >>> dividers, RCs, LRCs, things like that) and demonstrated that they >>> worked that way in real life. That was fun. >>> >>> What I was thinking lately was about making an LC oscillator with very >>> low phase noise, namely low jitter in my world. The finite Q of the >>> parallel LC is equivalent to a shunt resistor so I'd expect it to have >>> the Johnson noise of that equivalent resistance. Then the active stuff >>> must look like a negative resistor, which is noisy too. >> >> Yes, that's what I'd expect too. >> >>> >>> LT Spice noise analysis is very limited. I have sometimes added some >>> random-noise BV blocks in series with resistors and such, so I can do >>> genuine nonlinear sims with noise. It's actually easier to breadboard. >>> >> >> BTDT. What's with the nonlinear bit? LTspice noise analysis is >> basically an AC analysis, no? What irritates me is, that I need to edit .noise and .ac commands if I want to see noise and some gains; I want want both at the same time. The way it's now is clumsy. > LT Spice noise analysis is weird. You need one signal source, even if > you don't use it. And you can only probe one node. It must be entirely > linear. At least, that seems to work. < https://www.flickr.com/photos/137684711@N07/51974219045/in/album-72157662535945536/lightbox/ > > > I want to simulate jitter in a LC oscillator, and of course an > oscillator always has some nonlinear amplitude limiting mechanism. > If you want nonlinear noise, you need Keysight's Advanced Design System or such. Be prepared to a 5 or 6 digit price tag, depending on options. The keyword is harmonic balance simulator. cheers, Gerhard